In recent years, following the conductor pad pitch narrowing of integrated circuit (IC) chips, high density mounting on printed wiring boards has been advanced. Referring to FIG. 44A, a via conductor 50 is formed so as to cover a resin 60 and be coupled to an annular portion 511 of a through hole conductor 51. Assuming that an inner diameter of the through hole conductor 51 is D51 and a via diameter of the via conductor 50 is D50, the via conductor 50 is formed to establish a relationship of D50 is greater than D51. This is for connecting the via conductor 50 to the through hole conductor 51 to achieve electrical continuity therebetween.
For high density mounting on printed wiring boards, it is preferable to reduce the size of the inner diameter D51 of the through hole conductor 51 and the via diameter D50 of the via conductor 50. A reduction in the diameter of the via conductor 50 is possible, but a reduction in the diameter of the through hole conductor 51 is difficult to accomplish. Therefore, based on the fact that it is difficult to reduce the inner diameter D51 of the through hole conductor 51, but is possible to reduce the via diameter D50 of the via conductor 50, a via conductor 50 satisfying a relationship of D50 is less than D51 can be formed as shown in FIG. 44B. The reduction in the diameter of the via conductor 50 enables high density mounting on the printed wiring board. In this case, however, the via conductor 50 is not coupled to the through hole conductor 51, and therefore, no electrical continuity is achieved therebetween.
As a method for solving the foregoing problem, FIG. 44C shows a method of forming a capped conductor (plating) 52 covering the resin 60 and coupled to the annular portion 511 of the through hole conductor 51, and further forming the via conductor 50 on capped conductor 52. In this case, even if D50 is less than D51, the via conductor 50 is electrically coupled to the through hole conductor 51 via capped conductor 52. Consequently, the via conductor 50 can be reduced in size to thereby enable high density mounting on the printed wiring board.
However, the production of the printed wiring board having capped conductor 52 includes, after forming the through hole conductor 51 in a through hole 400, processes of filling the through hole 400 where the through hole conductor 51 is formed with the resin 60, and then forming capped conductor 52. Therefore, the following problems arise.
(1) When the through hole where the through hole conductor is formed is filled with a thermosetting resin in the form of a film using the lamination method, an unnecessary part of the resin can not be removed.
Generally, the filling of resin into the through hole where the through hole conductor is formed is carried out using a screen printing method or lamination method. However, when manufacturing the printed wiring board having a capped conductor, the filling of resin by the lamination method can not be employed. As shown in FIG. 45, in the lamination method, a process of filling the through hole 400 where the through hole conductor 51 is formed with the resin 60, and a process of forming a resin layer 80 on the surface of a base substrate 70 are simultaneously carried out. Therefore, the resin layer 80 is formed so as to cover the through hole conductor 51, and consequently, a capped conductor can not be formed.
(2) When performing the filling of resin by the screen printing method, it is not possible to carry out circuit pattern formation using the additive method.
In the case of filling of resin into the through hole where the through hole conductor is formed is implemented by the screen printing method, when the circuit pattern formation is carried out using the additive method, a process of plating the surface of the through hole 400 with copper to form the through hole conductor 51 and a process of forming a circuit pattern 90 are simultaneously carried out as shown in FIG. 46A. Subsequently, as shown in FIG. 46B, the through hole 400 where the through hole conductor 51 is formed is filled with the resin 60 by the screen printing method. After the filling and drying, as shown in FIG. 46C, part of the resin 60 projecting from the surface of the annular portion 511 of the through hole conductor 51 is abraded using a belt sander or the like. Upon abrasion, it is difficult to abrade only the resin over the surface of the annular portion 511, and therefore, the circuit pattern 90 formed in the vicinity of an opening of the through hole conductor 51 is also abraded by the belt sander resulting in damage to the circuit pattern 90. Consequently, when the filling of resin is implemented by the screen printing method, the circuit pattern formation by the additive method can not be carried out.
(3) When performing the filling of resin by the screen printing method, the accuracy of a circuit pattern can not be ensured if the circuit pattern formation is carried out by the subtractive method.
In the case of filling of resin into the through hole where the through hole conductor is formed is implemented by the screen printing method, when the circuit pattern formation is carried out using the subtractive method, having a copper foil 72 on the surface and formed with a through hole 73 is first prepared as shown in FIG. 47A. Then, as shown in FIG. 47B, a copper plating layer 74 is formed on the surface of the through hole 73 and the surface of the copper foil 72. In this event, the copper plating layer 74 formed on the surface of the through hole 73 assumes a cylindrical shape. Further, the through hole 73 where the copper plating layer 74 is formed is filled with the resin 60 by the screen printing method and, after drying, the resin 60 is abraded. After the abrasion, a copper plating layer 75 is formed on the resin 60 and the copper plating layer 74 as shown in FIG. 47C. After forming the copper plating layer 75, a circuit pattern 90 is formed by the subtractive method as shown in FIG. 47D. Simultaneously, the through hole conductor 51 and capped conductor 52 are also formed.
Inasmuch as the circuit pattern 90 is formed through the foregoing processes, it is composed of three layers having the copper foil 72, the copper plating layer 74, and the copper plating layer 75. As a result, the thickness of the circuit pattern 90 can not be made thin. If a thickness T of the circuit pattern 90 is large, the circuit pattern 90 assumes a trapezoidal shape in cross-section as shown in FIG. 47D, so that the accuracy of its shape can not be ensured and formation of a fine circuit pattern is prevented.
(4) capped conductor (plating) can not be formed having a relatively thick thickness.
In the case of implementing the filling of resin by the screen printing method, when the circuit pattern formation is carried out by the subtractive method, the circuit pattern 90 should be formed having a thin thickness as described above. Therefore, capped conductor 52 in the form of the copper plating layer 75 can not be formed thick. Consequently, deformation caused by differences in thermal expansion coefficients among the resin 60, the through hole conductor 51, and the base substrate 71 exerts an influence on a via conductor (not illustrated) formed on capped conductor 52.